As is well known, Stirling cycle cryogenic refrigerators, or cryocoolers, use a motor driven compressor to impart a cyclical volume variation in a working space filled with pressurized refrigeration gas. The pressurized refrigeration gas is fed from the compressor working volume through a heat exchanger assembly to an expansion working volume to which is attached the cold head. The heat exchanger assembly is made up of a heat exchanger located in the cold head, a regenerator, and another heat exchanger located adjacent to the compressor. The regenerator has openings in either end to allow the refrigeration gas to enter and exit.
The compressor and expander reciprocate in a fixed relationship creating the volume variations in the working space necessary to impart the Stirling cycle, and the refrigeration gas is forced to flow through the heat exchanger assembly in alternating directions. As the components reciprocate, the heat exchanger which directly receives the refrigeration gas from the compressor becomes much warmer than the ambient. In the other heat exchanger, attached to the expansion space, the gas is much colder than ambient. The device to be cooled is mounted adjacent the expansion space.
Because the cryocooler is sealed, the volume of the expansion and compressor spaces varies as the expander and compressor pistons reciprocate. The efficiency of a Stirling cryocooler is optimized by properly timing the movement of the expander and compressor pistons. Specifically, the component movements should be such that the variations in the volume of the expansion space lead the variations in the volume f the compression space by approximately 90.degree.. This insures that the compressor space pressure and temperature are at a peak before the refrigeration gas enters the regenerator from the warm end heat exchanger. To be cost effective Stirling cryocoolers must have long, maintenance free operating lives.
The two most common configurations of Stirling cryocoolers are referred to as "split" and "integral". The split Stirling type has a compressor which is mechanically isolated from the expander. Cyclically varying pressurized gas is fed between the compressor and expander through a gas transfer line. In most split Stirling cryocoolers proper timing of expander movement is achieved by using precision friction seals.
In an integral Stirling cryocooler, the compressor, heat exchangers, and expander are assembled in a common housing. The typical arrangement uses an electric motor to drive the moving parts. A crankshaft, disposed in a crankcase, is used to properly time compressor and expander movement, much as an internal combustion engine uses a crankshaft to provide proper timing of the movement of its pistons. As such, the typical integral cryocooler requires several bearings to support the crankshaft. If connecting rods are used to couple the compressor and expander to the crankshaft, additional bearings are required. A problem with this arrangement is that these bearings require lubricant. Also, lubricants are subject to freezing at cryogenic temperatures causing flow blockage within the regenerator reducing performance of the cryocooler. One way to eliminate the problem caused by lubrication is to seal the oil containing refrigerant gas in the crankcase from the oil-free refrigerant gas in the compressor and expander. Many different sealing arrangement have been used. Some Stirling systems use contact seals of the wearing type. However, these arrangement produce wear particles, which result in limited operating life. Other systems use elastomeric roll sock seals, which are complex, expensive and do not produce consistent life time results.
Further, other systems use a plurality of complicated bellows seal located within the Stirling Cycle work space, coupled with auxiliary pressure compensator seals which are located outboard of the bellows seal whereby the bellows seal is connected through a pump piston and a power piston simultaneously, as shown in U.S. Pat. No. 4,532,766. However, pressure pulsations inherent in the Stirling Cycle will cause unacceptable pressure differences across a single bellows seal located within the Stirling Cycle work space leading to high bellows material stresses and short operating life.